A piezoelectric actuator causes an electromechanical conversion element to expand and contract with a driving signal and produces progressive vibration waves (hereinafter referred to as progressive waves) at a driving surface of an elastic body with the expansion and contraction. The piezoelectric actuator produces elliptical motions at the driving surface with the progressive waves and drives a relative movement member in pressure contact with wave crests to output a driving force (see Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Application, Publication No. S58-148682